1. Field of the Invention
This invention relates to the design and preparation of semiconductor materials from materials which form an inclusion complex such as a clathrate compound or a zeolite having a crystal lattice type structure and more specifically filling cavities or cages associated with the crystal lattice type structure with selected atoms to enhance various thermoelectric properties of the semiconductor materials.
2. Description of the Related Art
The basic theory and operation of thermoelectric devices has been developed for many years. Presently available thermoelectric devices used for cooling typically include an array of thermocouples which operate in accordance with the Peltier effect. Thermoelectric devices may also be used for heating, power generation and temperature sensing.
Thermoelectric devices may be described as essentially small heat pumps which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other apparatus used to transfer heat energy. A principal difference is that thermoelectric devices function with solid state electrical components (thermoelectric elements or thermocouples) as compared to more traditional mechanical/fluid heating and cooling components. The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric Figure of Merit (ZT) of materials used in fabrication of the associated thermoelectric elements. Materials used to fabricate other components such as electrical connections, hot plates and cold plates may also affect the overall efficiency of the resulting thermoelectric device.
The thermoelectric Figure of Merit (ZT) is a dimensionless measure of the effectiveness of a thermoelectric device and is related to material properties by the following equation: EQU ZT=S.sup.2.sigma.T/.kappa. (1)
where S,.sigma.,.kappa., and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. The Seebeck coefficient (S) is a measure of how readily the respective carriers (electrons or holes) can transfer energy as they move through a thermoelectric element which is subjected to a temperature gradient. The type of carrier (electron or hole) is a function of the materials selected to form each thermoelectric element.
The electrical properties (sometimes referred to as electrical characteristics, electronic properties, or electronic characteristics) associated with materials used to form thermoelectric elements may be represented by S.sup.2.sigma.. Many of the materials which are used to form thermoelectric elements may be generally described as semiconductor compounds or semiconductor materials. The thermoelectric Figure of Merit is also related to the strength of interactions between the carriers and vibrations of the crystal lattice structure associated with semiconductor materials and available carrier energy states. Such vibrations may sometimes be referred to as "phonons".
Both the crystal lattice structure and the carrier energy states are a function of the materials selected to form each thermoelectric device. As a result, thermal conductivity (.kappa.) is a function of both an electronic component (.kappa..sub.e) primarily associated with the respective carriers and a lattice component (.kappa..sub.g) primarily associated with the respective crystal lattice structure and propagation of phonons through the respective crystal lattice structure. In general, thermal conductivity may be stated by the equation: EQU .kappa.=e.kappa..sub.e +.kappa..sub.g (2)
The thermoelectric Figure of Merit (ZT) may also be stated by the equation: ##EQU1##
Thermoelectric materials such as alloys of Bi.sub.2 Te.sub.3, PbTe and BiSb were developed thirty to forty years ago. The lattice component of thermal conductivity (K.sub.g) for such materials was reduced by forming a mixed crystal lattice type structure. More recently, semiconductor materials such as SiGe have been used in the fabrication of thermoelectric devices. Commercially available thermoelectric materials are generally limited to use in a temperature range between 300.degree. K. and 1300.degree. K. with a maximum ZT value of approximately one. The efficiency of such thermoelectric devices remains relatively low at approximately five to eight percent (5-8%) energy conversion efficiency. For the temperature range of -100.degree. C. to 1000.degree. C., maximum ZT for many state of the art thermoelectric materials also remains limited to values of approximately 1, except for Te-Ag-Ge-Sb alloys (TAGS) which may achieve a ZT of 1.2 to 1.4 in a relatively narrow temperature range. Recently developed materials such as Si.sub.80 Ge.sub.20 alloys used in thermoelectric generators to power spacecrafts for deep space missions have an average thermoelectric Figure of Merit approximately equal to 0.5 from 300.degree. C. to 1,000.degree. C.
An inclusion complex may be described as an unbonded association of a first material component and a second material component in which atoms of the first material component form a complex structure with cavities or cages. Atoms or molecules of the second material component are either wholly or partially trapped within the cavities or cages formed by the first material component. There are several types of inclusion complexes, including clathrates and zeolites. A typical clathrate compound such as 3C.sub.6 H.sub.4 (OH).sub.2.multidot.SO.sub.2 may be depicted as: ##STR1##
where the interlocked rings represent mutual enclosures of two cages. The formula for any clathrate compound is determined in part by the ratio of available cavities or cages to the amount of cage material.
Zeolite compounds may be described as a hydrated silicate of aluminum and either sodium or calcium or both, having a general formula of Na.sub.2 O.multidot.Al.sub.2 O.sub.3.multidot.xSiO.sub.2.multidot.xH.sub.2 O. Potassium compounds may also be present in zeolites. Both natural and artificial zeolites are used extensively for water softening, as detergent builders, and petroleum cracking catalysts. Natural zeolites include analcite, chabazite, heulandite, natrolite, stilbite, and thomosonite. Synthetic zeolites may be made by a gel process (sodium silicate and alumina) or a clay process (kaolin). The effectiveness of zeolites as a catalyst or filter will often depend upon the pore size, which may be as small as four to five angstroms. Other applications for zeolites include adsorbents, desiccants, and solar collectors, where zeolites may function as both heating and cooling agents.
Materials which form an inclusion complex have previously been used to separate molecules of different shapes, e.g., straight-chain hydrocarbons from those containing side chains, as well as structural isomers. Inclusion complex materials have also been used as templates for directing chemical reactions. Also, a wide variety of clathrate hydrates have previously been studied in which water (H.sub.2 O) (first material component of the inclusion complex) forms complex ice crystals with various atoms or molecules (second material component of the inclusion complex) trapped in the ice crystals.
Some disordered crystalline dielectric materials possess very low values of thermal conductivity (.kappa.) over a large temperature range, similar to the thermal conductivity of amorphous solids. These materials are known to have a number of common characteristics. For example, the presence of atoms or molecular groups having two or more semi-stable positions, and the absence of long-range correlation between their positions or orientations. The relationship between glass-like values of thermal conductivity (.kappa.) and the theoretical minimum thermal conductivity (.kappa..sub.min) is also known.
Various clathrate compounds and techniques for fabricating Me.sub.8 X.sub.46 and Me.sub.24 X.sub.136 with X.dbd.Si of Ge clathrate compounds are disclosed in a 1970 thesis entitled "Sur Quelques Nouveaux Siliciures et Germaniures Alcalins a Structure Clathrate: Etude Crystallochimique et Physique" by Christian Cross, University of Bordeaux, Bordeaux, France.